The 5-mercapto-1,2,3-thiadiazole salt represented by the above general formula (I) obtainable by using the method of this invention are important compounds as modifier of cephalosporins [see G. S. Lewis and P. H. Nelson, J. Med. Chem., 1979, 22, 1214 and Japnese Patent Application (OPI) No. 59895/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") which corresponds to published unexamined British Patent Application No. 2083461 and Belgian Patent 88913].
Heretofore, the followings have been considered to be the best method for preparing 5-mercapto-1,2,3-thiadiazole salts: haloacetaldehyde is allowed to react with a semicarbazide or an alkoxycarbonylhydrazine to afford the corresponding hydrazone, then the hydrazone is converted to 5-chloro-1,2,3-thiadiazole by the action of thionyl chloride and the thiadiazole is treated with sodium sulfide to give a desired thiadiazole salt (see West German Patent Application (OLS) Nos. 2,636,994 and 2,856,404).
However, this method contains a drawback in that 5-chloro-1,2,3-thiadiazole formed as an intermediate is explosive (see S. Morisaki, Thermochim. Acta., 1981, 47, 85). Accordingly, considerable difficulties are involved with treatment and handling.